1. Title of the Invention
The present invention relates to a load sensor that detects a load in response to an applied pressure ranging up to a predetermined load, and a robot apparatus having the load sensor provided at each of leg portions thereof, for example.
This application claims the priority of the Japanese Patent Application No. 2003-070721 filed on Mar. 14, 2003 and No. 2003-067769 filed on Mar. 13, 2003, the entirety of which is incorporated by reference herein.
2. Description of the Related Art
Recently, the research and development on legged robot apparatuses modeled on bipedal animals walking in erect posture such as the human being, monkey or the like have made remarkable advance, and the demand for practical applications of such robot apparatuses has become larger and larger. In the meantime, the bipedal robot apparatuses walking in erect posture are not so stable as a crawler-, quadruped- and hexapod-type ones and control of their posture and walking needs a complicated mechanism. However, they are excellent in smooth and flexible motion on a rough terrain or an obstacle, irregular surface of a work route in a construction site or the like, discontinuous walking surfaces of a stair or ladder, etc.
Most of the working and living spaces are tailored to the physical mechanism and behavior of the human being walking in erect posture on two feet. In other words, the space where the human being is living includes too many barriers against the current mechanical system using wheels and other drive units as a moving means. To make various jobs in support, or on behalf, of the human being and to be further accepted in the daily life of the human being, the mechanical system, or the robot apparatus, should desirably be able to move in a range nearly equal to the moving range of the human being. This is the reason for the large demand for the practical applications of the legged robot apparatuses. Being of a humanoid type is essential for the robot apparatus to have a higher affinity to the residential environment of the human being.
There have been proposed many techniques for posture control and stable walking of legged robot apparatuses of a bipedal walking type. Most of them adopts the ZMP (zero moment point) criterion for determination of the walking stability. This ZMP criterion is based on the d'Alembert's principle. This d'Alembert's principle is such that a weight and inertial force and their moment from a walking system will act on a road surface and a floor reaction force and floor reaction moment as reactions of the road surface on the walking system will balance with each other. It is inferred by the dynamics that there exists on or inside sides of a supporting polygon defined by the grounding point of a foot bottom (sole portion) and the road surface a point where the pitch-axis and roll-axis moments (floor reaction moment during walking) are zero. This point is called “ZMP”. Also, the ZMP trajectory is an orbit depicted by ZMP while a robot apparatus is walking, for example.
Control of bipedal walking on the basis of the ZMP criterion is advantageous in that a grounding point of the sole portion can be determined in advance and the kinematic constraint of the foot toe corresponding to the shape of a road surface can easily be considered. Also, since taking ZMP as the criterion for determination of the walking stability means taking an orbit, not any force, as the target value of an action control, the technical feasibility of the walking control can be improved. The above concept of ZMP and employment of ZMP as the criterion for determination of the walking stability a bipedal walking robot apparatus are referred to in the “Legged Locomotion Robots” by Miomir Vukobratovic.
For controlling the action of the robot apparatus using ZMP as the criterion for determination of the walking stability, measurement of an actual ZMP is very useful. On this account, a plurality of ZMP detecting sensors including a load sensor, acceleration sensor, etc. is disposed at a foot provided at the end of each moving leg of the robot apparatus (also see the Japanese Published Unexampled Application No. 1995-280671). A value detected by each of these sensors is converted from analog to digital, and the digital signal is supplied to a main controller provided in the body of the robot apparatus. The main controller calculates an actual ZMP on the basis of such detected values, and uses the calculation results for controlling each of actions including the walking of the robot apparatus.
Some of the conventional bipedal walking robot apparatuses use a double-structure foot formed from an instep member and a sole member installed to the instep member to be freely movable. The double-structure foot has many advantages in that the sole member can be changed correspondingly to the shape of a road surface and also replaced when it has been worn out by abrasion, and the leg and foot sole can be manufactured separately, for example. In this case, the ZMP detecting sensors are disposed between the instep and sole members. In case the ZMP detecting sensors (load sensor) are provided as above, however, when the robot apparatus walks on the aforementioned discontinuous walking surfaces, the sole member will bend or be otherwise deformed so that the load sensor will be applied with an extremely large load, thus be deteriorated in accuracy of detection or broken. Therefore, the sole member of the robot apparatus using such a double-structure foot has a stopper function incorporated in the whole foot, especially, in an exterior member of the foot, to prevent a large load from being applied to the load sensor.
However, since a plurality of load sensors has to be provided in the aforementioned double-structure foot, in case the stopper function corresponding to each load sensor is to be provided in the exterior member of the foot, the positioning accuracy of the load sensors should usually be in the order of tens micrometers (μm) and the stopper function has to be adjusted for all the load sensors. Namely, the requirement for the dimensional accuracy makes it extremely difficult to manufacture a robot apparatus using such double-structure feet. Thus, it becomes necessary to develop a foot structure having a stopper function incorporated in other than the sole member of the robot apparatus.
On the other hand, there has been proposed a load sensor that detects a load applied from above or below and has provided therein a mechanical stopper to prevent a diaphragm inside the load sensor from being broken by an excessive or impact load. The load sensor is schematically illustrated in FIG. 1. As shown, the load sensor includes a diaphragm 411 whose central portion is formed as a thin pressure-sensitive portion 411a on which strain gauges are provided, a rubber-made driving member 417 provided on the pressure-sensitive portion 411a, and an activating member 414 surrounding the driving member 417.
The activating member 414 is formed for a gap to occur between itself and the diagram 411 when it is applied with no load. When a load is applied to the activating member 414, a strain develops in the pressure-sensitive portion 411a of the diaphragm 411 via the driving member 417 and the load is thus detected by the strain gauges. When a larger load than a predetermined one is applied to the activating member 414, the latter will abut, at the bottom thereof, a thick portion 411b of the diaphragm 411 and prevent the pressure-sensitive portion 411a from being applied with any larger load. Thus, it is possible to protect the pressure-sensitive portion 411a against an excessive or impact load.
However, use of the above-mentioned load sensor in the foot of a robot apparatus will lead to the following problem. That is, since the aforementioned conventional load sensor has the rubber-made driving member provided between the activating member and diaphragm, the dimensional accuracy of the rubber and change of the ambient temperature will cause the load detection to vary from one sensor to another. Namely, the conventional load sensor cannot detect a load with a high accuracy. More particularly, although the load sensor is designed for the activating member to abut the diaphragm when applied with a predetermined load, a variation, if any, caused by the above factors will lead to a variation of the load under which the activating member is to abut with the diaphragm. The rubber will hardly displace linearly in response to any load applied. It will displace largely under a small load and will little displace under a certain magnitude of load. Therefore, the pressure-sensitive portion of the diaphragm is applied with a very large load even when the driving member slightly varies in characteristic, which is likely to cause the diaphragm to be broken in some cases. For calculation of ZMP in the robot apparatus, all the load sensors provided in the foot should be able to detect a load with a high accuracy. With the rubber-made driving member, however, it is extremely difficult to detect a load stably even with a large variation of the environmental condition.
As will be known from the above, the stopper provided in other than the exterior member of the foot allows an easy positioning, and it is necessary to develop a foot structure having provided therein load sensors whose load detection will not vary from one sensor to another.